Bifunctional air electrodes for an iron-air battery generally consist of three components. These are a hydrophobic membrane which permits air passage while retaining electrolyte, a single, fiber metal, nickel plaque, which serves as current collector and conductor and as a holder of a catalytic active material paste, and the catalytic active material paste composite itself, which is attached and pressed onto the current collector.
Such as iron-air battery is taught by Buzzelli, in U.S. Pat. No. 3,977,901. There, a polytetrafluoroethylene hydrophobic membrane was laminated to a single, 45 mil thick, 94 percent porous, sintered, nickel fiber current collector, which had an active material comprising acetylene black carbon and tungsten disulfide catalyst compact pasted onto its surface.
Due to the relative high viscosity of the active material paste, it has been necessary to apply considerable force during the pasting operation, in order to get reasonable loading. This force generally tends to compress and compact the sintered, fiber metal plaque structure, to provide only about a 40% to 65% porous plaque, and prevents the paste from filling all of the original plaque pores or voids. Loading generally ranges from about 15% to 35%; and electrode life, to 300 mV. below Hg/HgO, for single ply 45 mil thick plaques, having about 25 mg/sq. cm. loading, is about 615 hours.
The ideal incorporation of the catalytic paste into the plaque would be to have about 95% to 100% of the plaque pores filled. This would produce the highest number of active sites within the confines of the plaque, with all active sites very close to the metal current collecting fibers. It is also advantageous to have most of the active material within the plaque, rather than forming a separate composite layer held or attached to the plaque structure. Thick coatings of catalytic paste on the surface of the plaque result in many of the active sites being far removed from the nickel fibers, with electrons from these sites having to traverse a high resistance path to reach the current collector.
What is needed is an uncompacted air electrode that permits maximum catalytic paste loading and minimum resistance paths between active sites and current collecting and conducting fibers.